1. Field of the Invention
The invention relates to a method of manufacturing a fibrous cellulose casing and a casing manufactured by this method.
2. Description of Prior Art
Artificial cellulose casings, fibrous cellulose casings and fiber reinforced casings are manufactured from a long fiber paper base impregnated with cellulose which has been regenerated from viscose. Prior to viscose impregnation the paper is wet strengthened by the paper manufacturer, also using cellulose regenerated from viscose or alternatively using other wet strengthening materials such as polyamide epihalohydrin resins, polyvinyl alcohol, hydroxyethyl cellulose and various synthetically produced lattices either singly or in combination one with another. The chief reason for conferring this wet strength is to enable the paper to retain its integrity and withstand the subsequent heavy impregnation with the strongly alkaline viscose, which is applied by the casing manufacturer, and the subsequent strongly acidic cellulose coagulation and regeneration stage by which the casing is made.
Whereas the paper manufacturer may apply only one to six percent regenerated cellulose from viscose and/or other wet-strengthening resins calculated on the basis of total fiber weight to provide the necessary wet strength, the casing manufacturer usually applies a quantity of viscose of two to three times the total weight of the paper to construct the casing. For example, in the case of paper which weighs 21 g/m2, the casing manufacturer would add an additional 42 to 63 g/2 regenerated cellulose from viscose in making the casing, amounting to a total cellulose weight 63 to 84 g/m2. Fiber-reinforced cellulose of this example would be commercially prepared or plasticized with a quantity of 20 to 25 percent glycerol and moisturized with 5 to 10 percent water, to prevent it drying out and becoming brittle prior to use, achieving an overall basis weight given of between 78,8 and 119 g/m2.
The process for making the paper and the process for making casing are very different. For example, imparting wet-strength to the paper is relatively easy and inexpensive for the paper manufacturer, and may be carried on in-line, whereas wet strengthening is totally impracticable and prohibitively expensive for the casing manufacturer who requires the paper fully wet-strengthened from the outset of the casing manufacturing process.
The fiber of the paper substrate used to make fibrous casing is usually a carefully selected choice of abaca fibers, also known as manila hemp fibers, grown in either the Philippines or Ecuador, which are subjected to a chemical pulping treatment to remove non-cellulosic material and to soften the fibers for paper-making. A portion of this fiber may be provided by alternative plant fiber such as sisal, jute, or wood pulp, etc., but not without affecting the casing strength characteristics, usually in a negative way. Papers so prepared, of basis weight between 17 and 28 g/m2, are well known in the art for manufacturing fibrous cellulose casings, in the range of size typically from 32 to 163 mm in diameter, or even larger, and usually the basis weight of the paper is increased, for example, from 17 to 19 g/m2, from 19 to 21 g/m2, from 21 to 23 g/m2, and so on in stages, as diameter increases and the need for greater strength increases. This is because the weight of the sausage product increases exponentially as a function of a squaring of the radius of the sausage.
The casing manufacturer""s process usually involves unwinding the paper, supplied in rolls by the paper manufacturer, from an unwind stand, by drawing and forming the paper into the shape of a tube with overlapping margins. The paper is drawn continuously about its longitudinal axis by bending actions with the assistance of metal guides, while sticking the margins together using viscose immediately prior to the impregnation with bulk viscose which makes up the reinforced cellulose wall of the casing.
The composition of the aqueous viscose solution varies but may comprise, for example, 7 to 8 percent (wt-%) cellulose, 30-33 percent (wt-%) sodium cellulose xanthate of xanthate sulphur, 4-5 percent (wt-%) sodium hydroxide and water, with a ball-fall viscosity of 50 to 70 seconds (130 mg steel ball of 3,175 mm diameter over a distance of 20 cm). A Hottenroth ripening (salt) index of 4 to 5 at a temperature of 25 to 30 degrees Celsius may be regarded typical.
Once impregnated, the viscose is coagulated and regenerated into cellulose by passing the impregnated tube through a sulfuric acid and salt bath, usually containing ammonium and sodium sulphate mixtures, followed by various acid and water wash baths to complete the regeneration and remove all remaining sulphur from the viscose. Acid strength may vary from 40 to 60 g/l and a salt strength between 180 and 260 g/l with respect to sodium sulphate and between 10 and 50 g/l with respect to ammonium sulphate may be regarded typical. Before drying this fiber-reinforced tube it is usually passed through a bath containing a dilute aqueous glycerol solution, of 10-20 percent strength, to act as a plasticizer for the cellulose. Drying is preferably conducted with the casing in an inflated condition. Tension throughout the casing machine is maintained such that the diameter of the casing at the outset, that is the diameter provided by the original width of paper in the roll, less that used for overlapping, is reproduced as far as possible in the finished casing tube.
Viscose impregnation may be effected by pouring the viscose onto the outer surface of the paper substrate, well ahead of the acid bath, in order to ensure that the paper is thoroughly impregnated prior to regeneration of the cellulose with acid. Alternatively, the paper-impregnation may be accomplished with a specially constructed viscose die, wherein the viscose is presented under pressure, through the lips of the die, directly onto the outer surface of the paper or substrate. The paper is temporarily supported during transit across the ??? lips by a metal ring or cylinder, the gap between ring and die being such as to allow the paper to pass unimpeded, but not of such a width as to fully dissipate the pressure of the viscose before impregnating the paper. Impregnation is completed by passing paper between the die and the supporting ring for some distance within fixed gap dimensions, and thereafter passing the paper for some distance unsupported through air before entering the acid coagulation and regeneration bath.
Such impregnations may result in a product in which the two surfaces of the paper are unequally viscosed, only one side being fully impregnated owing to the resistance to penetration of papers of 17 g/m2 and greater; greater paper weight usually equating to greater resistance.
In still another variant, viscose may be supplied to both sides of the paper simultaneously in order to effect a so-called double-viscose coating. This ensures that the paper is properly covered by viscose prior to coagulation and regeneration of cellulose.
Cellulosic tubings produced in these various ways are tough and strong and have low stretch characteristics. For these reasons they may be used as containers or casings for sausages, meats, or other articles of food, particularly in applications where size, in terms of diameter control, is a critical parameter, and where a highly mechanized sausage stuffing plant places high demands for consistency of performance, and where strength with toughness is at a premium.
While exact size is achieved from a fibrous casing exhibiting low stretch, this also means that for differences in sausage diameter of only a few millimeters, another size of viscose die has to be used. This entails additional costs for the separate inventorying on the part of the paper supplier, the casing manufacturer and the sausage maker and for additional dies.
In parallel with the development of ever more demanding packaging equipment in technologically advanced countries, which as described above has placed ever greater demands on the casing producer to make tougher, stronger and more-size consistent casing (which demands have been met by the fibrous cellulose casing manufactures), collagen and cellophane casings, or pure cellulose casings unreinforced by a fibrous substrate have been developed for less critical market areas where exact size is not the first or most important selection criterion, but rather flexibility of use and unit cost are more important considerations.
These developments have been divergent rather than convergent, which is to say a gap exists between the two types of casing markets. For example, a highly flexible casing of consistent though by no means exceptional strength maybe required for mechanized stuffing machinery without subsequent packaging equipment also demanding exact size. In this example collagen casing would have provided the right price level but would not have provided the strength and associated low produce breakage rate characteristic of a fibrous casing, and the fibrous casing would be considered over-engineered and too costly.
Prior art improvements in casing manufacture usually focused on improving the wet strength or reducing the variability of strength of paper substrates typically of the order of 20 to 23 g/m2, so that a more consistent casing size or strength could be achieved. The use of polyamide-epihalohydrin resin and cationic polyethylene imine resin to provide an improvement in the alkaline wet tensile paper properties, in the U.S. Pat. No. 4,222,821 is one such example.
Another example is U.S. Pat. No. 3,433,633 which seeks to improve the quality of the viscose-bonded or wet-strengthened manila hemp paper substrate for subsequent fibrous cellulose casing manufacture by using cellulose derivatives of high viscosity to provide stronger casings. The implication of U.S Pat. No. 3,433,633 is that the existing 22.7 g/m2 substrates lack sufficient strength, and therefore teach away from the use of lower substrate paper basis papers.
In recent years patent specifications have sought to apply wet strengthening systems other than those based on viscose to treat the fibrous abaca fibers by employing a variety of resin-based systems. U.S. Pat. No. 5,063,104 filed in 1990 is an example which highlights how physical properties of paper may be affected in a dramatic or significant manner depending upon what wet strengthening binder or bonding method is substituted for viscose. One property which is adversely affected by the use of resins as wet-strengthening agents is paper stretch or elongation. Reductions of up to 20 to 50 percent for wet elongation, as compared to elongation properties for viscose bonded paper, were demonstrated in this patent. Fibrous casing elongation is very much dependent on the substrate paper""s elongation.
In order to make fibrous casings of ever-increasing diameter and therefore ever greater strength, the use of ever-increasing substrate paper basis weights and/or the use of strength adjuncts to complement viscose bonded casings, typically manufactured from paper having weights of 17 g/m2 and greater, had been required. This trend of using heavier weight paper to make wider diameter casings has as a corollary the fact that small diameter casings may be manufactured using papers with lower basis weight. However, in using low weight papers a limit is approached by virtue of the fact that a certain minimum paper strength in both dry and wet states is required so that the paper will withstand draw tension applied when it is unwound from its roll (dry strength), and thereafter possess sufficient strength, following impregnation with the viscose of the casing manufacturer""s process, so that it may successfully be converted into the casing product (wet strength).
Following the impregnation, the substrate paper tube, now saturated with viscose, has minimum strength and spends much of its time during its subsequent passage through the casing manufacturing machine as a flattened tube. The greater the surface area of this tube over turn-over rolls, which are used in the machine to convey it, and not all of which are necessarily driven, the less paper strength is required. These requirements for strength may be considered at variance: Manufacture of larger diameter casings using lighter weight substrate paper is relatively easy due to the large surface areas involved, but these casings do not perform robustly due to the high strength demands associated with large diameter casings compared to the low strength of the lighter-weight papers. Conversely, manufacture of narrower casings must be accomplished with heavier paper due to the relatively low surface areas involved, but narrow diameter casings may provide more strength than is required of the sausage-maker and be considered over-engineered and expensive.
In order to obtain the desirable physical property mix of the present invention using resin-bonded papers having lower wet extension or elongation properties compared to papers wet strengthened with viscose (prior to the viscose impregnation applied by the casing manufacturer), it had been found that substrate paper weights need to be reduced from 13 to 15 g/m2 to preferably 10 to 12 g/m2 when manufacturing processes are employed. Use of these light papers in conventional viscose dies (engineered with robust and heavy papers in mind) causes an undesirable xe2x80x9cnecking-inxe2x80x9d of the casings necking-in is brought about by a simultaneous extension of the tube in its longitudinal direction and a commensurate shrinkage in its transverse direction, which reduces the diameters of the casings from those defined by the die size. Absent steps taken to overcome this obstacle, the process of reducing paper weight to obtain more casing trans-directional extension becomes self-defeating: In order to overcome these difficulties while without reengineering the viscose dies used and designed for heavier paper substrates it is necessary to slow down the entire casing manufacturing operation thereby to reduce stress on the lightweight paper. This approach is generally not convenient or economically feasible in that additional costs are likely to be incurred as a result of slow processing.